Thanks to all for the info in this thread and additionally for the newly-uploaded PDF document. My bellows do move but look to be constrained by the non-movement of the part at the other end of the attached rod. It must be full of crud. Squeezing the pipe does reveal some movement in the topmost bellow but not much. It's probably been stuck like this since I bought the car back in 2011. I was aware of the repair to the upper bellow as it stuck out like a sore thumb but wasn't sure of how it operated correctly. I took a photo of this (see attachment) as mine appears to have been 'repaired' at some point by persons unknown.

I now feel confident at having a go at repairing this myself, armed with the proper documentation. Thanks again. If my puny arm strength lets me down, I might entrust this to the local garage. Just out of interest (once this is repaired/fixed), does this device yield a noticeable improvement in power/torque further up the rev. range?

Update: just RTFM so I know how it should work in theory.

Still curious as to the repair carried out on the top bellow though, as per photo.

I think the bolt is moulded into the end of the bellows, and perhaps the end of the bellows has fractured/broken, and then been repaired in the way shown.